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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering – Stage 3
(EMECH_8_Y3)
Autumn 2008
Manufacturing Engineering
(Time: 3 Hours)
Instructions: Answer FIVE questions Answer TWO from each of Sections A and B and ONE from section C Use a separate answer book for each section
Examiners: Mr. M P. O’Mahony Dr. M. Woods Mr. D. Fallon Mr. P. Clarke Prof. M. Gilchrist
Section A
Q1. (a) List FIVE Casting Parameters (5 Marks)
(b) Chvorinov’s Rule states
n
s A
t C V
Explain the terms used in the equation (2 Marks)
(c) The total solidification times for three casting shapes are to be compared: a sphere, a cylinder, in which the length to diameter ratio is = 1.0 and a cube. For all three geometries, the volume = 1000cm^3. The same casting alloy is used in the three cases. Take n= (i) Determine the relative solidification times for each geometry (ii) Based on (i) above which geometric element would make the best riser (iii) If the mold constant is = 3.5 min/cm^2 what is the total solidification time for each casting (8 Marks)
(d) Explain Directional Solidification and how it is achieved in the casting process (5 Marks)
Q2. (a) List FIVE advantages and/or disadvantages of thread rolling (5 Marks)
(b) Briefly describe ONE method of Thread grinding (5 Marks)
(c) A special fine thread has a nominal outside diameter of 4 inches. The pitch is 0.25in and the threads are of Whitworth Form. The thread is acceptable if the effective diameter is between nominal size and minus 0.008 in. When checked with wires of diameter 0.15in the dimensions over wires was 4.070 in. If the flank angle and pitch are correct, calculate the effective diameter and hence state whether the thread is acceptable. (Note Height of fundamental triangle for Whitworth H= 0.9605P) (8 Marks)
(d) When measuring the effective diameter of a thread, explain what is meant by the term “Best Wire” (2 Marks)
Section B
Q4. (a) (i) What is diffusion? (ii) Name THREE factors which affect the rate of diffusion. (iii) Compare interstitial and substitutional diffusion in terms of : the role of defects; the type of atoms involved; energetics; and speed. (10 marks) (b) (i) State the equation for Fick’s 1 st^ Law and explain all terms of the equation. (ii) A sheet of iron of thickness 0.85mm is used to contain nitrogen in a heat exchanger. The concentration of nitrogen atoms on one side of the sheet is 3.75 x 10^29 m- and on the other side it is 5.0 x 10^28 m-3^. Calculate the atomic flux per metre squared (J) and the mass flux per metre squared (M) of nitrogen through the iron. The molar mass of nitrogen, m =14.0 g mol-1^ ; Avogadro’s constant, A = 6.02 x 10^23 mol-1^ ; The diffusivity of nitrogen in iron, D = 4.50 x 10-11^ m 2 s -1^ ). (10 marks)
Q5. (a) (i) Describe a simple experimental method of determining the toughness (G (^) c) of the adhesive of a roll of sellotape of width t. Derive an equation for the toughness based on the experimental parameters. (ii) A 5 cm wide carpet tape has an adhesive with a toughness of 700 J m-2^. What is the minimum load force required to peel the tape? (8 marks) (b) Describe with the aid of a diagram the cleavage process of fast fracture. State the materials which undergo this process and theoretical values of strain ε and breaking stress σb at which it occurs. (6 marks)
(c) State the fast fracture condition. Hence calculate the stress ( σ ) required to cause fast
fracture in a concrete floor with a crack length a = 120 μm, and a fracture toughness K (^) c = 0.22 MN m-3/2^. (6 marks)
Q6. (a) (i) Discuss the microscopic origins of friction between two metal surfaces.
(ii) Prove, using a simple model, which considers the relevant stresses involved, that a Static Coefficient of Friction value of μs = ½ is obtained for two horizontal metal surfaces in contact with a load R and an applied force P attempting to move one surface relative to the other. (8 marks) (b) Describe the operation and frictional properties of the original type of skis made from wood. How are the operational problems overcome in modern skis? (6 marks) (c) State the optimum temperature conditions of fuel consumption and the effect of temperature on performance (output power) for turbine engines. How are turbine blades designed to be able to attain these conditions without being subject to creep? (6 marks)
